The traditional procedure for bonding an indirect restoration, such as an inlay, onlay, or crown to a tooth structure is rather cumbersome. First the carious dentition is removed. The tooth is etched with an acidic etchant to remove the smear layer on the tooth surface, and the tooth is then coated with a thin layer of an acidic primer. The base and catalyst of a cement is mixed and applied to the bonding surface of the indirect restoration, and the restoration is then seated onto the primed tooth surface.
Cement hardening may be effected by self-curing by incorporating a self-cure initiator system, or by the combination of self-curing and photo-curing by using a photo-initiator. When metal is involved in the restorative material, such as a porcelain fused to a metal (PFM) crown, an efficient self-curing cement is needed because it is difficult for light to penetrate the restorative material and reach the cement with sufficient intensity to achieve adequate photo-curing.
The bonding procedure may be simplified by using a self-etching primer, eliminating the etch step, and thus accomplishing etching and priming in one step. The bonding procedure is simplified into the steps of applying a self-etching primer to the tooth and then applying a cement to the restoration material.
To further simplify the bonding procedure, it would be highly desirable to combine the steps of etching, priming, and cementing into one step. This would significantly shorten the chair time for certain restorative procedures. This would also significantly reduce the possibility for errors involved in the restorative procedure because only one step cementation would be involved.
Great difficulties, however, are encountered when combining the ingredients necessary to fulfill the functions of etching, priming, and cementing. For example, acidic compounds are used as an etchant or an etching primer, but the self-cure initiator commonly used in a resin cement is not effective under strongly acidic environments. The self-cure initiator system currently used by most manufacturers to effect self-curing of a resin cement comprises benzoyl peroxide and an aromatic tertiary amine. Two problems arise with use of this system. First, the aromatic tertiary amine loses its effectiveness immediately upon contact with a moderate or strong acid. Second, benzoyl peroxide is not very stable in acidic environments, quickly loses its potency and, therefore, has a rather short shelf-life. As a result, when a self-etching primer is incorporated into a resin cement, the mixed cement fails to harden because the redox initiators lose their potency under acidic environments. A simplified system which reduces or eliminates these difficulties is needed.